The pH scale is a scale from 0 to 14 that indicates the acidity of a substance. The number 7 is in the middle and represents neutrality, in terms of qualifying a substance as an acid or a base. Substances that are lower than 7 are classified as acids, meaning they have increasing numbers of hydrogen ions (H+) when dissolved with water. Substances that are higher than 7 are classified as bases, meaning they have less hydrogen ions and more hydroxide ions (OH-) when dissolved with water.

To find the pH of a .1 M sample of H3PO4, understand the .1 M is the same as 1 x 10-1. If we take the logarithm of 1 x 10-1, that would be expressed as - (-1), which would be 1. The pH of this solution would be 1, which would qualify it as a very strong acid, providing many hydrogen ions upon dissolution with water. Th pH scale is very useful in indicating the acidity or basicity of substances.